It has already been proposed, when a correlation exists between the characteristics of the fluid medium and its refractive index, to detect the changes in these characteristics by detecting variations of this refractive index by means of various optical methods. Most of these optical methods are based on exploiting the reflection and refraction phenomena which occur near the critical angle. They essentially consist in transmitting light through a transparent light-conducting structure immersed in the fluid medium, so that light undergoes multiple internal reflections on the walls of the structure. The determination of the intensity of the light thus transmitted by multiple reflections and the sudden variation of this intensity near the critical angle thus permits the refractive index of the fluid to be determined.
To make continuous index measurements there are, for example, devices of the type consisting of a straight transparent rod with an opto-mechanical system at one end for injecting a pencil of light into the rod with a well-defined angle of incidence, and with a photo-electric detector at its other end for measuring the intensity of the light thus transmitted through the rod by multiple internal reflections with a well-defined angle of incidence. When the rod is immersed in the fluid medium to be measured, the angle of incidence of the pencil of light injected into the rod is then made to decrease continuously while observing the transmitted light intensity and the sudden drop in intensity which occurs when the angle of incidence of the multiple relfections exceeds the critical angle with respect to the fluid permits this critical angle to be determined and hence the refractive index of the fluid. Devices of this type however have the major drawback of being extremely complicated, given that they require, among other things, a relatively sophisticated light injection system, since it must ensure both a parallel pencil of incident light by optical means, and a continuous variation of the angle of incidence of this pencil by mechanical means.
Moreover, various known devices for measuring liquid levels are of the type comprising a prism (or cone) placed on the bottom end of a transparent rod inserted into the vessel containing the liquid of which the level is to be determined, indication of this level being obtained by injecting light into the top end of the rod and by observing visually the light reflected by the prism back to the top end (this end is illuminated by reflected light in the absence of liquid at the level of the prism, and becomes dark in the opposite case). Yet devices of this sort have a certain number of drawbacks: firstly the small number of reflections which occur (simple or double reflection) only allows a low light contrast to be obtained, while the light transmission factor remains relatively low; the structure of such devices has moreover proved relatively complicated. But these devices exhibit, above all, the major drawback of being able to function in only two different fluid states due to the fixed incidence of light, so that they are hardly suitable for carrying out continuous index measurements and their use thereby remains limited almost exclusively to the detection of changes of state such as level indication.
In order to remedy the above-mentioned drawbacks, it has further been proposed to use devices consisting of simple transparent rods comprising an intermediate U-shaped curved section adapted for immersion in the liquid to be tested, the refractive index of this fluid being determined by injection of light at one end of the rod and by observing the light transmitted to its other end. In such devices, the curved section of the rod results in the passage by refraction into the liquid of an amount of light which is found to be essentially a function of the refractive index of this liquid, so that the quantity of light transmitted to the other end of the rod constitutes a parameter which is characteristic of this refractive index (a device of this type is described for example in the article "A photo-electric refractometer" by E. Karrer and R. Orr--Journal of the Optical Society of America--Volume 36 No. 1--pages 42 to 46--January 1946). Such devices appear a priori to be particularly advantageous due to their great simplicity and low cost, as well as the fact that they may be used in principle for the detection of both discontinuous and continuous changes in the characteristics of the liquid to be tested. However, these devices exhibit the major drawback of having a very low sensitivity, so that their use as refractometers is quite limited, (because of their inability to be able to detect slight variations of the refractive index of the liquid to be tested), while even their use as simple liquid-level indicators is found to be far from satisfactory (because of the low contrast which can be measured).
It has been proposed more recently to provide different variants of these devices consisting of curved transparent rods, but none of the variants proposed up to the present time has led to a notable improvement of their sensitivity. Thus, for example, it has been proposed to replace the U-shaped rod by a rod with a curvature of at least 360.degree. (U.S. Pat. No. 3,282,149), but this simply serves to linearize the measurement, without notably changing the sensitivity. It has also been proposed, for example, to replace the transparent rod by a curved optical fiber (French Pat. No. 2,130,037) essentially in order to achieve a miniaturization of the equipment, but this mere replacement also has practically no effect on the sensitivity.